Au-Si Droplets Research Articles

Stability and energetics of two-dimensional surface crystals in liquid AuSi thin films and nanoscale droplets

Segregation at surfaces of metal-covalent binary liquids is often non-classical and in extreme cases such as AuSi, the surface crystallizes above the melting point. In this study, we employ atomic-scale computational frameworks to study the surface crystallization of AuSi films and droplets as a function of composition, temperature and size. For temperatures in the range $T_s^\ast=765-780$K above the melting point $(T_s^\ast\approx1.3\,T_m)$, both thin film and droplet surfaces undergo a first order transition, from a 2D Au$_2$Si crystalline phase to a laterally disordered yet stratified layer. The thin film surfaces exhibit an effective surface tension that increases with temperature and decreases with Si concentration. On the other hand, for droplets in the size range $10-30$ nm, the bulk Laplace pressure alters the surface segregation as it occurs with respect to a strained bulk. Above $T_s^\ast$ the size effect on the surface tension is small, while for $T<T_s^\ast$ the surface layer is strained and composed of 2D crystallites separated by extended grain boundary scars that lead to large fluctuations in its energetics. As a specific application, all-atom simulations of AuSi droplets on Si(111) substrate subject to Si surface flux show that the supersaturation dependent surface tension destabilizes the contact line via formation of a precursor wetting film on the solid-vapor interface, and has ramifications for size selection during VLS-based routes for nanowire growth. Our study sheds light on the interplay between stability and energetics of surfaces in these unique class of binary alloys and offers pathways for exploiting their surface structure for varied applications such as catalytic nanocrystal growth, dealloying, and polymer crystallization.

In-Plane Si Nanowire Growth Mechanism in Absence of External Si Flux.

We report on a new mechanism of nanowire formation: during Au deposition on Si(110) substrates, Au-Si droplets grow, move spontaneously, and fabricate a Si nanowire behind them in the absence of Si external flux. Nanowires are formed by Si dissolved from the substrate at the advancing front of the droplets and transported backward to the crystallization front. The droplet shape is determined by the Si etching anisotropy. The nanowire formation can be tuned by changing experimental parameters like substrate temperature and Au deposition rate.

Self-propelled motion of Au–Si droplets on Si(111) mediated by monoatomic step dissolution

By Low Energy Electron Microscopy, we show that the spontaneous motion of gold droplets on silicon (111) is chemically driven: the droplets tend to dissolve silicon monoatomic steps to reach the temperature-dependent Au–Si equilibrium stoichiometry. According to the droplet size, the motion details are different. In the first stages of Au deposition small droplets nucleate at steps and move continuously on single terraces. The droplets temporarily pin at each step they meet during their motion. During pinning, the growing droplets become supersaturated in Au. They depin from the steps when a notch nucleate on the upper step. Then the droplets climb up and locally dissolve the Si steps, leaving behind them deep tracks formed by notched steps. Measurements of the dissolution rate and the displacement lengths enable us to describe quantitatively the motion mechanism, also in terms of anisotropy of Si dissolution kinetics. Scaling laws for the droplet position as a function of time are proposed: x ∝ tn with 1/3<n<2/3.

Au transport in catalyst coarsening and Si nanowire formation.

The motion of Au between AuSi liquid eutectic droplets, both before and during vapor-liquid-solid growth, is important in controlling tapering and diameter uniformity in Si nanowires. We measure the kinetics of coarsening of AuSi droplets on Si(001) and Si(111), quantifying the size evolution of droplets during annealing in ultrahigh vacuum using in situ transmission electron microscopy. For individual droplets, we show that coarsening kinetics are modified when disilane or oxygen is added: coarsening rates increase in the presence of disilane but decrease in oxygen. Matching droplet size measurements on Si(001) with coarsening models confirms that Au transport is driven by capillary forces and that the kinetic coefficients depend on the gas environment present. We suggest that the gas effects are qualitatively similar whether transport is attachment limited or diffusion limited. These results provide insight into manipulating nanowire morphologies for advanced device fabrication.

Growth mechanisms of vapor–liquid–solid grown nanowires: A detailed analysis of irregular nanowire formation

Although vapor–liquid–solid (VLS) growth has become a standard method for producing nanowires, the underlying growth mechanisms have not been fully elucidated because VLS growth is affected by thermodynamic and geometrical factors that, to date, have mainly been considered separately. Based on the assumption that the irregular nanowire growth reflects the conditions of the eutectic droplet during nanowire growth, we aimed to elucidate the thermodynamic and geometrical aspects of the growth mechanisms by the measurement and analysis of irregular kinked and curved Si nanowires grown by Au-catalyzed VLS. The results suggest that kinked nanowires may be caused by higher supersaturation, whereas curved nanowires may be caused by lower supersaturation within the eutectic Au–Si droplet. The results of measuring and analyzing irregular nanowires confirmed that both thermodynamic and geometrical effects complexly influence nanowire growth kinetics, particularly the supersaturation of eutectic droplets, and the effects are more pronounced in larger diameter nanowires.

Atomistics of vapour-liquid-solid nanowire growth.

Vapour–liquid–solid route and its variants are routinely used for scalable synthesis of semiconducting nanowires, yet the fundamental growth processes remain unknown. Here we employ atomic-scale computations based on model potentials to study the stability and growth of gold-catalysed silicon nanowires. Equilibrium studies uncover segregation at the solid-like surface of the catalyst particle, a liquid AuSi droplet, and a silicon-rich droplet–nanowire interface enveloped by heterogeneous truncating facets. Supersaturation of the droplets leads to rapid one-dimensional growth on the truncating facets and much slower nucleation-controlled two-dimensional growth on the main facet. Surface diffusion is suppressed and the excess Si flux occurs through the droplet bulk which, together with the Si-rich interface and contact line, lowers the nucleation barrier on the main facet. The ensuing step flow is modified by Au diffusion away from the step edges. Our study highlights key interfacial characteristics for morphological and compositional control of semiconducting nanowire arrays.

Behavior of Au-Si droplets in Si(001) at high temperatures

The transport behavior of Au-Si droplets near the Si(001) surface at elevated temperatures is investigated using transmission electron microscopy. It has been found that Au-Si droplets move differently under different temperatures, which lead to the formation of SiOx surface islands on top of droplets, and result in the lateral movements of smaller droplets away from their corresponding surface islands. Since Au droplets have been widely used as catalysts to induce semiconductor nanowires, this study provides insight behavior of Au containing droplets on semiconductor surfaces, which is critical for understanding the formation mechanisms of semiconductor nanowires.

Metastable phase formation in the Au-Si system via ultrafast nanocalorimetry

We have investigated the stability and solidification of nanometer size Au-Si droplets using an ultrafast heating/cooling nanocalorimetry and in situ growth techniques. The liquid can be supercooled to very low temperatures for both Au-rich (ΔT ∼ 95 K) and Si-rich (ΔT ∼ 220 K) samples. Solidification of a unique metastable phase δ1 is observed with a composition of 74 ± 4 at. % Au and a b-centered orthorhombic structure (a = 0.92, b = 0.72, and c = 1.35 nm; body-center in the a-c plane), which grows heteroepitaxially to Aus. Its melting temperature Tm is 305 ± 5 °C. There is competition during formation between the eutectic and δ1 phases but δ1 is the only metastable alloy observed. For small size droplets, both the δ1 and eutectic phases show considerable depression of the melting point (size-dependent melting).

The size and diffusion effect of gold on silicon nanowire sidewall faceting

Single crystalline silicon nanowires (SiNWs) were grown using a gold (Au)-catalyzed vapor-liquid-solid (VLS) approach. In this study, we examine the influence of the size of Au catalyst droplets on the size of SiNWs and discuss the effect of Au diffusion and surface passivation on SiNW sidewall faceting and roughening in the VLS process. To simultaneously cover a variety of sizes of SiNWs on the same substrate, 2-nm-thick Au film was used on Si (111) substrate as a catalyst, since it is known that Au thin film–based seed offers relatively less control of the NW size, due to the randomness of the film breakup at reaction temperature. We then found that the grown SiNWs have two main types of surface morphologies on the sidewall of the nanowires (NWs). One type had a small and coarse surface morphology with no Au-Si droplets at the top of the NWs. The other type had a long and smooth surface and still had Au-Si droplets at the end. The fact that resulting SiNWs have two main different surface morphologies can be explained by three reasons: the size difference of Au-Si droplets, Au diffusion to the surface of the SiNWs, and surface passivation on the SiNW surface. Furthermore, we proposed a combined VLS and vapor-solid (VS) growth model to describe the creation of the facets and roughening of the sidewall of the NWs. Our results point toward the exciting possibility that the proposed well-controlled and coarse-surfaced SiNWs could be promising thermoelectric materials with low thermal conductivity, since the thermal conductivity is greatly reduced at the rough surface due to the enhancement of phonon scattering.

The effect of pattern density and wire diameter on the growth rate of micron diameter silicon wires

Silicon microwires (SiMWs) were grown by the vapor–liquid–solid process using gold thin films and gold patterned Si substrates. By precisely controlling the volume of gold within each pore, one wire of the desired wire diameter can be grown from each pore for wires with diameters up to 3 μm. The growth rate was found to decrease with increasing wire density for a constant wire diameter. Additionally, the micron diameter wires exhibited a decreasing growth rate with increasing wire diameter. This was further confirmed by experiments carried out using a gold thin film, where the diameter-dependent growth rate was observed to change from increasing with wire diameter in the small wire diameter range to a reduction in the growth rate for wire diameters greater than 1.25 μm. The decrease in growth rate with diameter for large diameter wires was determined to arise from the time required for the Au–Si droplet to supersaturate once exposed to the SiCl 4 precursor, as confirmed by growths with durations shorter than the nucleation time.

Heterogeneous nucleation and depletion effect in nanowire growth

Recent experiments by Ross and co-workers proved the possibility of a mononuclear regime with heterogeneous nucleation as well as jerky growth in the vapor–liquid–solid (VLS) process for silicon nanowires. In this work, a theoretical model is presented which incorporates the effects of (i) a mononuclear regime with layer by layer growth, (i) heterogeneous nucleation of each new layer at the edge of a Au–Si droplet, (iii) drop of supersaturation after each successful nucleation and respective fast layer growth, (iv) time-dependent nucleation barrier during each new waiting period and (v) correlation between subsequent waiting periods (non-Markovian sequence of waiting periods).

Growth Time-Dependent Density and Surface Evolution of Silicon Nanowires in a Vapor–Liquid–Solid Process

Single crystalline silicon nanowires (SiNWs) were grown on Si(100) substrate using a gold (Au)-catalyzed vapor-liquid-solid (VLS) approach. The dependence of the growth time (i.e., the time of exposure to the Si source) on the density and surface evolution of the grown SiNWs is considered. It was observed that the density of grown SiNWs on Si substrate increased with increasing growth time. The highest density (approximately 1.1 x 10(6) mm(-2)) was reached at 4 hr. Upon further exposure to the Si source, we observed that the density was maintained for up to 9 hr. We suggest that the increased Si chemical potential in Au-Si droplets with increased growth time enhanced the SiNW growth rate at the interfaces between Au-Si droplets and SiNWs, and enhanced the transition of the NWs from the existing Au-Si droplets onto Si substrate. This allows the SiNW density to increase with increased growth time of up to 4 hr. Moreover, we examined the influence of the growth time on surface evolution including Au diffusion, facet and taper formation, and vapor-solid (VS) growth of the SiNWs. To explain the behavior of the grown SiNWs in the VLS process, we propose a combined model using the VLS and VS growth mechanisms.

Kinetics of Individual Nucleation Events Observed in Nanoscale Vapor-Liquid-Solid Growth

We measured the nucleation and growth kinetics of solid silicon (Si) from liquid gold-silicon (AuSi) catalyst particles as the Si supersaturation increased, which is the first step of the vapor-liquid-solid growth of nanowires. Quantitative measurements agree well with a kinetic model, providing a unified picture of the growth process. Nucleation is heterogeneous, occurring consistently at the edge of the AuSi droplet, yet it is intrinsic and highly reproducible. We studied the critical supersaturation required for nucleation and found no observable size effects, even for systems down to 12 nanometers in diameter. For applications in nanoscale technology, the reproducibility is essential, heterogeneity promises greater control of nucleation, and the absence of strong size effects simplifies process design.

Formation of epitaxial gold nanoislands on (100) silicon

Coherent gold nanoislands were prepared directly on (100)-oriented Si substrates by a physical methodology, consisting of the thermal evaporation of a very thin Au film $(t\ensuremath{\sim}2\text{ }\text{nm})$ and its successive annealing in the temperature range $350\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}lTl814\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. We found that at annealing temperature of $814\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and in the presence of residual oxygen during the annealing process, epitaxial monocrystalline gold nanoislands embedded in the Si lattice are formed. The crystallographic orientation and epitaxial relationship between the Au nanoislands and the Si lattice are well defined. In contrast, at lower annealing temperatures, namely at $350\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and $626\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, the nanoislands are randomly oriented without epitaxial relationships. The morphology, orientation, and crystalline structure of Au nanoislands were investigated by scanning and high-resolution transmission electron microscopy and grazing-incidence x-ray diffraction. A model of the epitaxial Au nanoisland formation on (100)Si is presented in which the Si-atom out-diffusion and the formation of a liquid Au-Si droplet during the annealing process (increasing temperature) and the Si redeposition and oxidation (i.e., ${\text{SiO}}_{x}$ complex formation and removing of the excess Si in the gold islands) during the cooling process (decreasing temperature) play a fundamental role.

Critical condition for growth of silicon nanowires

The existence of a critical radius on the growth of Si nanowires by the vapor-liquid-solid mechanism is examined. By varying the experimental growth parameters, we have shown a dependence of the minimum nanowires radius with the Si reactive species partial pressure, demonstrating that the critical radius is not a limited one. A thermodynamical model giving a quantitative aspect of the dependence of a critical nanowire diameter on Si supersaturation in a Au-Si droplet is proposed. These results open up a way to grow many kinds of nanowires with nanometric diameter. The size control has important implications for electronic and optical properties of nanowires based devices.

Directed movement of Au–Si droplets towards buried dislocation networks on silicon bicrystals

Directed movement of Au–Si alloy droplets towards buried dislocation grids on a Si bicrystal has been observed by in situ ultrahigh vacuum transmission electron microscopy. It was found that once the underlying dislocation structure was dissolved, the movement of Au–Si droplets was directed to the region with remaining dislocation network. The migration of Au–Si droplets is driven by the energy difference between the strained bicrystal and nonstrained single-crystal silicon. The directed movement by the buried dislocation network is potentially significant in a wide range of technologies.

Arrangement of Catalyst Islands at Surface Atomic Steps toward Position Control of Nanowires

Using low-energy electron microscopy we show that Au deposition leads to the arrangement of three-dimensional islands at surface atomic steps on Si(111). Because the islands nucleate within a narrow coverage window, they have a small size distribution and their size can be precisely controlled by the deposition time. During the Au deposition on the surface with a controlled island density, the islands move into upper terraces with trenches left behind. This indicates that the islands are Au-Si alloy droplets. The Au-Si alloy islands act as catalysts for the nanowire formation based on the vapor-liquid-solid (VLS) mechanism, which enables us to fabricate the one-dimensional alignment of vertical GaP nanowires. Controlling the size and position of the nanostructures by using surface atomic steps as templates means “top-down” lithographic techniques are not needed. Our method is a pure “bottom-up” self-assembly one and is expected to greatly contribute to establishing self-assembled nano-device fabrication methods.

Wetting of Si surfaces by Au–Si liquid alloys

The behavior of liquid Au–Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650 °C and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid–solid–vapor phase line. In addition, the behavior of Au–Si droplets on vicinal and patterned surfaces of Si has been examined. The droplets cause step bunching and modify the local surface structure. Solidification of the droplets on all surfaces leads to phase separation.

XPEEM Study of Liquid Au-Si Droplets on Si(111) near to the Eutectic Point

XPEEM Study of Liquid Au-Si Droplets on Si(111) near to the Eutectic Point